Meteor Radio Reflection Detection
- Matthew Earnshaw (M3MNH)
What is this about?
Every day, 2 billion meteoroids (Motz and Duveen 1976, McKinley 1961 suggests 200 million) of various sizes enter the Earth's atmosphere, most no bigger than a grain of sand. This page is dedicated to the monitoring of meteor activity by use of radio waves.
What's the point?
Organisations like RMOB and the IMO provide platforms whereby distributed data obtained by meteor observation stations such as this one can be collated. This gives an excellent base of information for meteor research. The SAO/NASA Astrophysics Data System shows how important the data is given the wealth of radio meteor studies found on the system. I contribute data to RMOB and The International RMO Project.
How does it work?
The purple/orange spots in the spectrogram on the homepage show radio waves being reflected off the ionised gas 'trail' left by meteors, most no bigger than a grain of sand and up to 2000km from the point of observation. Signal from a distant VHF TV transmitter in Eastern Europe on 59.258MHz is reflected off the ionised gas and that signal is received by a receiver listening on 59.258MHz. This is called radio forward scatter. The audio output of the receiver is then processed using the excellent free spectrum analyser, SpectrumLab. My script (written using SpectrumLab's conditional interpreter) outputs the spectrogram image, counts meteors, records detailed information about each meteor event, writes the data into RMOB format and takes pictures of exceptionally strong reflections for later analysis and review.
What does a meteor echo sound like and what can be inferred from it?
A meteor echo sounds like a short ping, typically lasting 0.3 to 1 second but up to a few minutes in exceptional cases (fireballs). Listen to my live broadcast and see if you can hear a meteor. The persistance of ionisation gives us a clue as to the mass/velocity of the meteroid. Meteoroids can enter the atmosphere at velocities up to 70km/second, so they have very high kinetic energy. The greater the kinetic energy, the greater the ionising potential of the meteoroid. From the image produced over time on the spectrogram, corresponding to the change in electron line density over time by ambipolar diffusion, atmospheric wind shear effects and local atmospheric densities could be infered. Eventually the free electrons produced in the ionisation trail recombine (with the positive ions) and then will stop "reflecting" radio waves. Although you can infer a lot from a meteor echo, it isn't trivial to translate radio observations to what would be seen visually because this is a very complex relationship.
What's a fireball?
Most meteors are faint ones, in fact only one tenth of the meteoroids that enter the atmosphere produce visible trails. Few meteors exceed magnitude 0, but 1 in 1000 or so might have magnitudes as high as -8 (the lower the number, the brighter the object, on a log scale). "Fireball" is the name given to exceptionally bright meteors. In radio meteor terms, an echo longer than 10 seconds is classed as a fireball. The Fireball gallery shows some fireballs detected during the 2009 Perseids. You can see how diffuse the ionisation the trail from fireballs can become by how much it spreads vertically on the spectrogram.
Where can I find more information?
International Meteor Organisation (IMO), Radio Meteor Observing Bulletin (RMOB). Or consult "Meteor Science and Engineering - D.W.R.McKinley, McGraw-Hill 1961". Any other questions, please contact me 
.